Power management circuit of rechargeable battery stack

ABSTRACT

A system including a plurality of cells connected in series in a rechargeable battery pack and a plurality of cell balancing modules. Each cell balancing module performs voltage balancing of a respective pair of cells. Each cell balancing module includes a communication module to (i) transmit, via a communication link, information about voltages of the respective pair of cells to an adjacent cell balancing module and (ii) receive, via the communication link, from the adjacent cell balancing module, information about voltages of cells corresponding to the adjacent cell balancing module. Each cell balancing module performs, based on the information received from the adjacent cell balancing module, the voltage balancing in response to a voltage difference between any of the plurality of cells being greater than or equal to a predetermined threshold instead of performing the voltage balancing based on a difference between voltages of the respective pair of cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 12/973,298(now U.S. Pat. No. 8,525,478), filed on Dec. 20, 2010, which claims thebenefit of U.S. Provisional Application No. 61/292,740, filed on Jan. 6,2010. The entire disclosures of the above applications are incorporatedherein by reference.

FIELD

The present disclosure relates to rechargeable batteries and moreparticularly to a power management circuit for a rechargeable batterystack.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Rechargeable batteries are used in many applications. The applicationsmay range from portable electronic devices to industrial equipment. Forexample, the portable electronic devices may include cell phones,cameras, personal digital assistants (PDAs), laptop computers, andnotebook computers. The industrial equipment may include fork-lifts,hybrid-electric vehicles, medical equipment, and uninterruptible powersupplies.

Rechargeable batteries typically include cells that utilize differentchemical technologies and that generate different output voltages. Forexample, Nickel-Cadmium (NiCd) and Nickel Metal Hydride (NiMH) cellsgenerate an output voltage of 1.2 volts (1.2V). Lithium ion cellsgenerate output voltages ranging from 3.6V to 3.9V.

Many applications utilize voltages that may be greater than the outputvoltage generated by a single cell. Accordingly, a battery stack ofmultiple cells may be used to generate output voltages that are greaterthan the voltage generated by a single cell. For example, a batterystack comprising two cells may generate an output voltage that can powersome portable electronic devices. A battery stack comprising hundreds ofcells may generate an output voltage that can power some electricvehicles.

SUMMARY

A system comprises a sensing module and a switching module. The sensingmodule is configured to sense output voltages of first and second cellsconnected in series in a rechargeable battery stack. The switchingmodule is configured to alternately connect a capacitance across thefirst cell and the second cell at a switching frequency when adifference in the output voltages is greater than or equal to a firstthreshold. The switching module is further configured to stopalternately connecting the capacitance when the difference is less thanor equal to a second threshold, wherein the first threshold is greaterthan the second threshold.

In other features, the system further comprises the first and secondcells and the capacitance to transfer charge between the first andsecond cells when the capacitance is alternately connected across thefirst and second cells at the switching frequency.

In another feature, an integrated circuit (IC) comprises the system.

In another feature, an integrated circuit (IC) comprises the system andfurther comprises the capacitance.

In another feature, by alternately connecting the capacitance, chargetransfer is effected between the first and second cells to preventovercharging or over-discharging.

In still other features, a system comprises N cells connected in seriesin a rechargeable battery stack, where N is an integer greater than 1and (N−1) cell balancing modules to balance output voltages of the Ncells. Each of the (N−1) cell balancing modules balances output voltagesof adjacent ones of the N cells. Each of the (N−1) cell balancingmodules includes a sensing module and a switching module. The sensingmodule is configured to sense the output voltages of the adjacent onesof the N cells. The switching module is configured to alternatelyconnect a capacitance across a first of the adjacent ones of the N cellsand a second of the adjacent ones of the N cells at a switchingfrequency when a difference in the output voltages of the adjacent onesof the N cells is greater than or equal to a first threshold. Theswitching module is further configured to stop alternately connectingthe capacitance when the difference is less than or equal to a secondthreshold, wherein the first threshold is greater than the secondthreshold.

In another feature, each of the (N−1) cell balancing modules furthercomprises the capacitance to transfer charge between the adjacent onesof the N cells when the capacitance is alternately connected across theadjacent ones of the N cells at the switching frequency.

In another feature, each of the (N−1) cell balancing modules isimplemented by an integrated circuit (IC).

In another feature, each of the (N−1) cell balancing modules isimplemented by an integrated circuit (IC).

In another feature, by alternately connecting the capacitance, chargetransfer is effected between the first and second of the adjacent onesof the N cells to prevent overcharging or over-discharging.

In still other features, a system comprises N cells connected in seriesin a rechargeable battery stack, where N is an integer greater than 1,and (N−1) cell balancing modules to balance output voltages of the Ncells. Each of the (N−1) cell balancing modules balances output voltagesof adjacent ones of the N cells. Each of the (N−1) cell balancingmodules includes a communication module and a switching module. Thecommunication module is configured to communicate the output voltages ofthe adjacent ones of the N cells to others of the (N−1) cell balancingmodules via a communication link. The switching module is configured toalternately connect a capacitance across a first of the adjacent ones ofthe N cells and a second of the adjacent ones of the N cells at aswitching frequency when a difference in output voltages of two of the Ncells is greater than or equal to a predetermined threshold.

In another feature, each of the (N−1) cell balancing modules furthercomprises a sensing module to sense the output voltages of the adjacentones of the N cells.

In another feature, each of the (N−1) cell balancing modules furthercomprises the capacitance to transfer charge between the adjacent onesof the N cells when the capacitance is alternately connected across theadjacent ones of the N cells at the switching frequency.

In other features, the system further comprises a stack control moduleconfigured to communicate with one of the (N−1) cell balancing modulesvia the communication link. The stack control module receives the outputvoltages of the N cells via the communication link. The stack controlmodule controls coupling of the rechargeable battery stack to a chargingsystem based on the output voltages of the N cells.

In another feature, the stack control module controls coupling of therechargeable battery stack to a load based on the output voltages of theN cells.

In another feature, by alternately connecting the capacitance, chargetransfer is effected between the first and second of the adjacent onesof the N cells to prevent overcharging or over-discharging.

Further areas of applicability of the present disclosure will becomeapparent from the detailed description, the claims and the drawings. Thedetailed description and specific examples are intended for purposes ofillustration only and are not intended to limit the scope of thedisclosure.

BRIEF DESCRIPTION OF DRAWINGS

The present disclosure will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1A is a functional block diagram of a system forcharging/discharging two adjacent cells of a battery stack;

FIG. 1B depicts an exemplary implementation of a cell balancing module;

FIG. 2A is a functional block diagram of a cell balancing moduleintegrated circuit (IC) comprising a capacitor;

FIG. 2B is a functional block diagram of a cell balancing module ICwithout a capacitor;

FIGS. 3A-3C are functional block diagrams of systems forcharging/discharging four cells of a battery stack;

FIGS. 3D and 3E are functional block diagrams of systems forcharging/discharging three and five cells of a battery stack,respectively;

FIG. 4A is a functional block diagram of a cell balancing module ICcomprising a communication module and a capacitor;

FIG. 4B is a functional block diagram of a cell balancing module ICwithout a capacitor and comprising a communication module;

FIGS. 5A-5B are functional block diagrams of systems forcharging/discharging a plurality of cells of a battery stack;

FIGS. 6A-6B are functional block diagrams of systems forconnecting/disconnecting a battery stack;

FIG. 7 is a flowchart of a method for charging/discharging cells of abattery stack using hysteresis;

FIG. 8 is a flowchart of a method for charging/discharging cells of abattery stack; and

FIG. 9 is a functional block diagram of a system forcharging/discharging two adjacent cells of a battery stack utilizing aninductive element.

DESCRIPTION

The following description is merely exemplary in nature and is in no wayintended to limit the disclosure, its application, or uses. For purposesof clarity, the same reference numbers will be used in the drawings toidentify similar elements. As used herein, the phrase at least one of A,B, and C should be construed to mean a logical (A or B or C), using anon-exclusive logical OR. It should be understood that steps within amethod may be executed in different order without altering theprinciples of the present disclosure.

As used herein, the term module may refer to, be part of, or include anApplication Specific Integrated Circuit (ASIC), an electronic circuit, aprocessor (shared, dedicated, or group) and/or memory (shared,dedicated, or group) that execute one or more software or firmwareprograms, a combinational logic circuit, and/or other suitablecomponents that provide the described functionality.

A cell of a battery has a capacity to store a predetermined amount ofcharge. The capacity may be called a rated capacity of the cell. Anamount of charge remaining in the cell at any time may be expressed interms of a state of charge of the cell. A cell is in a fully chargedstate when charged to its maximum capacity (e.g., the rated capacity).Conversely, a cell is in a fully discharged state when discharged to aminimum capacity. The output voltage of the cell is a function of thestate of charge of the cell.

Occasionally, a cell may be unable to store charge according to itsrated capacity. Instead, the cell may store less charge than its ratedcapacity. A cell may be called a weak cell or a strong cell based on itsability to store charge according to its rated capacity.

For example, a strong cell can store charge nearly equal to its ratedcapacity when fully charged. Conversely, a weak cell stores considerablyless charge than its rated capacity when fully charged.

When cells are connected in series in a battery stack, the same amountof current flows through the cells during charging and discharging.During charging, a weak cell charges faster than a strong cell and isfully charged before the strong cell. An output voltage of the weak cellreaches its maximum rated value before the strong cell. The weak cell isovercharged when charging is continued to fully charge the strong cell.The output voltage of the weak cell exceeds its maximum rated value whenthe weak cell is overcharged.

During discharging, the weak cell discharges faster than the strong celland is fully discharged before the strong cell. The output voltage ofthe weak cell decreases from its maximum rated value faster than thestrong cell. The strong cell may reverse charge the weak cell whendischarging is continued until the strong cell is fully discharged.

Frequent overcharging and reverse charging adversely impacts the numberof useful charge-recharge cycles of the cells. Most cells have limitednumber of useful charge-recharge cycles. For example, lead-acid cellsmay have 200-500 useful charge-recharge cycles. Nickel-Cadmium (NiCd)cells may have 500-1200 useful charge-recharge cycles. Lithium ion cellsmay have 300-500 useful charge-recharge cycles. The number of usefulcharge-recharge cycles is considerably reduced when the cells weaken andare overcharged for a prolonged period of time. Moreover, the cells maybe damaged when the weak cells are completely discharged and arereversed charged.

To prevent overcharging and over-discharging of the weak cells, thebattery stack may be operated at less than its rated capacity. Forexample, a charging cycle of the battery stack may be terminated whenthe weak cell is fully charged. Terminating the charging cycle when theweak cell is fully charged may prevent other cells in the battery stackfrom fully charging. As a result, the battery stack may supply lesspower than its rated capacity.

Conversely, a discharge cycle of the battery stack may be terminatedwhen the weak cell is fully discharged. Terminating the discharge cyclewhen the weak cell is fully discharged may prevent other cells in thebattery stack from fully discharging. As a result, the battery stack maysupply less power than its rated capacity.

Operating the battery stack at less than its rated capacity may resultin waste due to unused capacity of the battery stack. Additionally,operating the battery stack at less than its rated capacity may increasethe number of charge-recharge cycles.

Instead, each cell of the battery stack may be monitored individually.The charging and discharging of each cell may be controlled to preventdamage to the weak cells. For example, controllable dissipative bypassdevices may be used with each cell. A controller that controls chargingand discharging may sense when a weak cell is fully charged. Thecontroller may turn on a dissipative bypass device associated with theweak cell when the weak cell is fully charged. The dissipative bypassdevice prevents the weak cell from further charging while other cellsare allowed to charge to their full capacity. Thus, the dissipativebypass device prevents overcharging of the weak cell.

Additionally, the controller may sense when the weak cell is nearlyfully discharged. The controller may disable further discharging of theweak cell when the weak cell is nearly fully discharged. Thus, thecontroller may prevent over-discharging of the weak cell.

This approach protects the weak cells from being overcharged andover-discharged. However, the useful capacity of the strong cells is notmade available to the application. Further, using dissipative bypassdevices reduces round-trip charge/discharge efficiency during charging.

The present disclosure relates to using a capacitive charge transferbetween adjacent cells of a battery stack to equalize output voltages ofthe adjacent cells. Specifically, the output voltages of adjacent cellsmay be equalized using capacitive charge transfer by alternatelyconnecting a capacitor across a first cell and a second cell.

For example, the output voltage of the first cell may be greater thanthe output voltage of the second cell. The capacitor may charge whenconnected across the first cell and discharge when connected across thesecond cell. The capacitor may be connected alternately across the firstand second cells until the output voltages of the first and second cellsare equal.

Further, the capacitive charge transfer between two adjacent cells(hereinafter charge transfer) may be efficient when the charge transferis hysteretic. Specifically, the charge transfer may be initiated when adifference in the output voltages becomes greater than an upperthreshold. Subsequently, the charge transfer may be terminated when thedifference decreases and becomes less than or equal to a lowerthreshold.

More specifically, the efficiency of the charge transfer is inverselyproportional to the difference in the output voltages of adjacent cells.The efficiency is higher when the difference is smaller. Hence, toincrease the efficiency, the charge transfer may be initiated when thedifference becomes greater than the upper threshold.

Switching the capacitor between adjacent cells, however, dissipatespower. When the difference is less than or equal to the lower threshold,the power dissipated in switching the capacitor may outweigh thebenefits of reducing the difference further below the lower threshold.

Thus, reducing the difference below the lower threshold may decrease theefficiency of the charge transfer. Accordingly, the switching may beactivated when the difference exceeds the upper threshold anddeactivated when the difference reaches the lower threshold. In otherwords, the switching and the charge transfer may be hysteretic.

Referring now to FIG. 1A, a system 100 for balancing (i.e., nearlyequalizing) output voltages of cells in a rechargeable battery stack isshown. Since the system 100 balances the output voltages by capacitivecharge transfer, the system 100 may be called a capacitive voltagebalancing system. The charge transfer and voltage balancing may beperformed when the rechargeable battery stack is charging and/ordischarging.

The system 100 comprises a rechargeable battery stack 102 (hereinafterbattery stack 102) and a cell balancing module 104. For example only,the battery stack 102 comprises a first cell Cell-1 and a second cellCell-2. Cell-1 and Cell-2 are adjacent cells and are connected inseries. The cell balancing module 104 comprises a first switch SW1, asecond switch SW2, a capacitor C, a sensing module 106, and a switchingmodule 108.

The cell balancing module 104 controls the charging and discharging ofthe battery stack 102. The cell balancing module 104 is connected acrosstwo outside terminals 102-1, 102-2 of the battery stack 102. In someimplementations, the cell balancing module 104 may be integrated intothe battery stack 102.

The sensing module 106 is connected across the two outside terminals102-1, 102-2 of the battery stack 102. Additionally, the sensing module106 is connected to a node N1 that connects the Cell-1 and Cell-2 inseries. The sensing module 106 senses output voltages of Cell-1 andCell-2 and senses a difference between the output voltages (hereinaftervoltage difference).

During charging and discharging of the battery stack 102, the sensingmodule 106 activates the switching module 108 when the voltagedifference is greater than or equal to the upper threshold. The sensingmodule 106 deactivates the switching module 108 when the voltagedifference is less than or equal to the lower threshold. The upperthreshold is greater than the lower threshold.

When activated, the switching module 108 switches the capacitor Cbetween Cell-1 and Cell-2 at a predetermined frequency. Specifically,the switching module 108 alternately connects the capacitor C acrossCell-1 and Cell-2 at a high switching frequency. For example only, theswitching frequency may be in a range from 100 KHz to 1 MHz.

The switching module 108 may comprise a signal generator (not shown)that generates one or more signals having the predetermined switchingfrequency. The one or more signals may be used to change positions ofthe switches SW1 and SW2 at the predetermined switching frequency. Thecapacitor C is connected across Cell-1 when the positions are set toposition 1. The capacitor C is connected across Cell-2 when thepositions are set to position 2.

An average current through the capacitor C is C*ΔV*F, where C is thecapacitor value in Farad, ΔV is the voltage difference between Cell-1and Cell-2, and F is the switching frequency. For example only, if C=1μF, ΔV=0.1V, and F=1 MHz, the average current during the charge transferwill be approximately 0.1 A.

The first and second switches SW1 and SW2 may be implemented using anysemiconductor switching devices. For example only, the first and secondswitches SW1 and SW2 may be implemented using metal-oxide semiconductorfield-effect transistors (MOSFETs). The first and second switches SW1and SW2 may each have an on-state resistance (R_(on)). The capacitor Cis selected such that a time constant (R_(on)*C) is less than or equalto half the period of the switching frequency.

Referring now to FIG. 1B, an exemplary implementation of the switchesSW1 and SW2 is shown. The sensing module 106 is omitted so that theimplementation of the switches SW1 and SW2 can be clearly shown. Each ofthe switches SW1 and SW2 may comprise a negative MOSFET (NMOS)transistor and a positive MOSFET (PMOS) transistor. The gates of theNMOS and PMOS transistors of the switches SW1 and SW2 are driven by asignal generated by the switching module 108. The signal may have thepredetermined switching frequency.

Respective first ends (sources) of the NMOS and PMOS transistors of theswitch SW1 are connected to a first end of the capacitor C. Respectivefirst ends (sources) of the NMOS and PMOS transistors of the switch SW2are connected to a second end of the capacitor C. Respective second ends(drains) of the PMOS transistors of the switches SW1 and SW2 provideposition 1 of the switches SW1 and SW2. Respective second ends (drains)of the NMOS transistors of the switches SW1 and SW2 provide position 2of the switches SW1 and SW2.

The second end (drain) of the NMOS transistor of the switch SW1 isconnected to the second end (drain) of the PMOS transistor of the switchSW2. The second end of the NMOS transistor of the switch SW1 and thesecond end of the PMOS transistor of the switch SW2 are connected to thenode N1. The drain of the PMOS transistor of the switch SW1 and thedrain of the NMOS transistor of the switch SW2 are connected to the twooutside terminals 102-1, 102-2 of the battery stack 102, respectively.

When the signal generated by the switching module 108 is low, the NMOStransistors of the switches SW1 and SW2 turn off. The PMOS transistorsof the switches SW1 and SW2 turn on and connect the first and secondends of the capacitor C to position 1 of the switches SW1 and SW2. Thus,the capacitor C is connected across Cell-1 when the signal generated bythe switching module 108 is low.

When the signal generated by the switching module 108 is high, the PMOStransistors of the switches SW1 and SW2 turn off. The NMOS transistorsof the switches SW1 and SW2 turn on and connect the first and secondends of the capacitor C to position 2 of the switches SW1 and SW2. Thus,the capacitor C is connected across Cell-2 when the signal generated bythe switching module 108 is high.

The implementation shown in FIG. 1B is for example only. Many otherimplementations comprising other switching devices are contemplated. Theimplementations may comprise more or fewer number of transistors (orswitching devices) than those shown. Depending on the number ofswitching devices used, the switching module 108 may generate one ormore signals to turn the switching devices on and off. Further, in theimplementation shown, the connections of drains and sources may beinterchanged. Additionally, the NMOS and PMOS transistors may beinterchanged.

In use, for example, Cell-1 may be weaker than Cell-2. During chargingof the battery stack 102, the weak cell charges faster than the strongcell. The weak cell may be nearly fully charged while the strong cell isnot yet fully charged. The output voltage of the weak cell may begreater than the output voltage of the strong cell. A voltage differencemay exist between Cell-1 and Cell-2. The sensing module 106 senses thevoltage difference.

When the voltage difference is greater than or equal to the upperthreshold, the sensing module 106 activates the switching module 108.The switching module 108 switches the capacitor C alternately betweenCell-1 and Cell-2 at the switching frequency. The capacitor C transferscharge from the weak cell to the strong cell. Accordingly, the outputvoltage of the weak cell decreases while the strong cell continues tocharge. Thus, the capacitor C prevents the weak cell from overchargingwhile the strong cell charges to its full capacity.

As the strong cell charges to nearly full capacity, the voltagedifference between Cell-1 and Cell-2 decreases. The sensing module 106senses when the voltage difference decreases to less than or equal tothe lower threshold. The sensing module 106 deactivates the switchingmodule 108 when the voltage difference decreases to less than or equalto the lower threshold. The switching module 108 stops switching thecapacitor C alternately between Cell-1 and Cell-2.

During discharging of the battery stack 102, the weak cell dischargesfaster than the strong cell. The weak cell may be nearly fullydischarged while the strong cell is not yet fully discharged. The outputvoltage of the weak cell may be less than the output voltage of thestrong cell. A voltage difference may exist between Cell-1 and Cell-2.The sensing module 106 senses the voltage difference.

When the voltage difference is greater than or equal to the upperthreshold, the sensing module 106 activates the switching module 108.The switching module 108 switches the capacitor C alternately betweenCell-1 and Cell-2 at the switching frequency. The capacitor C transferscharge from the strong cell to the weak cell. The capacitor C preventsthe weak cell from over-discharging while the strong cell fullydischarges. Additionally, due to the charge transfer, the output voltageof the weak cell increases to a value greater than the output voltage ofthe strong cell. This prevents reverse-charging of the weak cell by thestrong cell.

The sensing module 106 senses when the voltage difference between Cell-1and Cell-2 decreases to less than or equal to the lower threshold. Thesensing module 106 deactivates the switching module 108 when the voltagedifference decreases to less than or equal to the lower threshold. Theswitching module 108 stops switching the capacitor C alternately betweenCell-1 and Cell-2.

Referring now to FIGS. 2A and 2B, the cell balancing module 104 can beimplemented in two ways. In one implementation, the cell balancingmodule 104 can be implemented by a 3-terminal integrated circuit (IC).In FIG. 2A, a cell balancing module IC 104 comprising the capacitor Cand having three terminals is shown.

Alternatively, if the capacitor C is not integrated in the same IC, thecell balancing module 104 without the capacitor C may be implemented bya 5-terminal IC. In FIG. 2B, a cell balancing module IC 105 without thecapacitor C and having five terminals is shown. The capacitor C isconnected externally to two of the five terminals as shown.

Generally, the battery stack 102 may include N cells, where N is aninteger greater than 1. One cell balancing module 104 may be used forevery two adjacent cells. Accordingly, (N−1) number of the cellbalancing module 104 may be used to balance the N cells of the batterystack 102.

Referring now to FIGS. 3A-3C, for example, various systems forcapacitive voltage balancing of four cells of the battery stack 102 areshown. In FIG. 3A, a schematic of a system 200 comprising three cellbalancing modules to balance four cells of the battery stack 102 isshown. In FIG. 3B, a functional block diagram of a system 200-1comprising the four cells and three cell balancing module ICs 104 isshown. In FIG. 3C, a functional block diagram of a system 200-2comprising the four cells and three cell balancing module ICs 105 isshown.

In FIG. 3A, the system 200 comprises first, second, and third cellbalancing modules 202, 204, 206 and Cell-1, Cell-2, Cell-3, and Cell-4of the battery stack 102. In the first, second, and third cell balancingmodules 202, 204, 206, only the capacitor C and the switches SW1 and SW2are shown. Other components of the first, second, and third cellbalancing modules 202, 204, 206 are assumed present and are omitted forclarity. The first, second, and third cell balancing modules 202, 204,206 balance the output voltages of Cell-1, Cell-2, Cell-3, and Cell-4.

Specifically, the first cell balancing module 202 balances the outputvoltages of Cell-1 and Cell-2. The second cell balancing module 204balances the output voltages of Cell-3 and Cell-4. The third cellbalancing module 206 balances the output voltages of Cell-2 and Cell-3.

The first cell balancing module 202 is connected across Cell-1 andCell-2 and communicates with node N1 that connects the Cell-1 and Cell-2in series. The second cell balancing module 204 is connected acrossCell-3 and Cell-4 and communicates with node N2 that connects the Cell-3and Cell-4 in series. The third cell balancing module 206 is connectedacross Cell-2 and Cell-3 and communicates with node N3 that connects theCell-2 and Cell-3 in series. The third cell balancing module 206 isconnected across the nodes N1 and N2.

In FIG. 3B, the system 200-1 comprises first, second, and third cellbalancing module ICs 104-1, 104-2, 104-3 and Cell-1, Cell-2, Cell-3, andCell-4 of the battery stack 102. The first, second, and third cellbalancing module ICs 104-1, 104-2, 104-3 balance the output voltages ofCell-1, Cell-2, Cell-3, and Cell-4.

Specifically, the first cell balancing module IC 104-1 balances theoutput voltages of Cell-1 and Cell-2. The second cell balancing moduleIC 104-2 balances the output voltages of Cell-3 and Cell-4. The thirdcell balancing module IC 104-3 balances the output voltages of Cell-2and Cell-3.

The first cell balancing module IC 104-1 is connected across Cell-1 andCell-2 and communicates with node N1 that connects the Cell-1 and Cell-2in series. The second cell balancing module IC 104-2 is connected acrossCell-3 and Cell-4 and communicates with node N2 that connects the Cell-3and Cell-4 in series. The third cell balancing module IC 104-3 isconnected across Cell-2 and Cell-3 and communicates with node N3 thatconnects the Cell-2 and Cell-3 in series. The third cell balancingmodule IC 104-3 is connected across the nodes N1 and N2.

In FIG. 3C, the system 200-2 comprises first, second, and third cellbalancing module ICs 105-1, 105-2, 105-3 and Cell-1, Cell-2, Cell-3, andCell-4 of the battery stack 102. The first, second, and third cellbalancing module ICs 105-1, 105-2, 105-3 balance the output voltages ofCell-1, Cell-2, Cell-3, and Cell-4.

Specifically, the first cell balancing module IC 105-1 balances theoutput voltages of Cell-1 and Cell-2. The second cell balancing moduleIC 105-2 balances the output voltages of Cell-3 and Cell-4. The thirdcell balancing module IC 105-3 balances the output voltages of Cell-2and Cell-3.

The first cell balancing module IC 105-1 is connected across Cell-1 andCell-2 and communicates with node N1 that connects the Cell-1 and Cell-2in series. The second cell balancing module IC 105-2 is connected acrossCell-3 and Cell-4 and communicates with node N2 that connects the Cell-3and Cell-4 in series. The third cell balancing module IC 105-3 isconnected across Cell-2 and Cell-3 and communicates with node N3 thatconnects the Cell-2 and Cell-3 in series. The third cell balancingmodule IC 105-3 is connected across the nodes N1 and N2.

Referring now to FIGS. 3D and 3E, in some implementations, a batterystack may include N cells, where N is an odd integer greater than 1.When the number of cells in the battery stack is odd, one of the cellsin a pair balanced by one cell balancing module and one of the cells ina pair balanced by an adjacent cell balancing module may be the same.

In FIG. 3D, for example only, suppose that a battery stack includesthree cells: first, second, and third cells (Cell-1, Cell-2, andCell-3). A first cell balancing module 150-1 may balance output voltagesof the first and second cells (Cell-1 and Cell-2) while a second cellbalancing module 150-2 may balance output voltages of the second andthird cells (Cell-2 and Cell-3). Here, the second cell (Cell-2) is thesame in the pairs of cells balanced by both the first and second cellbalancing modules 150-1 and 150-2. Each of the cell balancing modules150-1 and 150-2 may be either the cell balancing module IC 104 or thecell balancing module IC 105.

In FIG. 3E, as another example, suppose that a battery stack includesfive cells: first, second, third, fourth, and fifth cells (Cell-1,Cell-2, Cell-3, Cell-4, and Cell-5). A first cell balancing module 155-1may balance output voltages of the first and second cells (Cell-1 andCell-2). A second cell balancing module 155-2 may balance outputvoltages of the third and fourth cells (Cell-3 and Cell-4). A third cellbalancing module 155-3 may balance output voltages of the second andthird cells (Cell-2 and Cell-3). A fourth cell balancing module 155-4may balance output voltages of the fourth and fifth cells (Cell-4 andCell-5).

Here, the second cell (Cell-2) is the same in the pairs of cellsbalanced by both the first and third cell balancing modules 155-1 and155-3. The third cell (Cell-3) is the same in the pairs of cellsbalanced by both the second and third cell balancing modules 155-2 and155-3. The fourth cell (Cell-4) is the same in the pairs of cellsbalanced by both the second and fourth cell balancing modules 155-2 and155-4. Each of the cell balancing modules 155-1, 155-2, 155-3, and 155-4may be either the cell balancing module IC 104 or the cell balancingmodule IC 105.

In some implementations, the number of cells (N) in the battery stack102 may be high. That is, N may be a large number. For example only, Nmay be of the order of 10, 100, or higher. When N is large, if voltagebalancing is performed based on the voltage difference between twoadjacent cells, the voltage difference between distant cells can becomevery high. For example, the voltage difference between the first celland the last cell in the battery stack 102 may become very high.

When the battery stack 102 comprises a very large number of cells, thecell balancing modules may be linked using communication links. The cellbalancing modules may communicate voltage states of the cells in thebattery stack 102 via the communication links. The voltage states mayinclude output voltages and/or differences in the output voltages of thecells balanced by the cell balancing modules. Based on the voltagestates, the cell balancing modules may perform capacitive voltagebalancing when the voltage difference between any two cells is greaterthan a threshold.

To facilitate the communication links, the cell balancing modules mayeach comprise a communication module. Additionally, to selectivelyperform capacitive voltage balancing, the cell balancing modules mayeach comprise a control module. Accordingly, the cell balancing moduleICs 104 and 105 may be modified as follows.

Referring now to FIGS. 4A and 4B, cell balancing module ICs comprising acommunication module and a control module are shown. In FIG. 4A, a cellbalancing module IC 300 comprises all the components of the cellbalancing module IC 104. Additionally, the cell balancing module IC 300comprises a communication module 302 and a control module 304.

The communication module 302 communicates with the sensing module 106.The sensing module 106 senses output voltages of adjacent cells (e.g.,Cell-1 and Cell-2) and determines voltage states of the adjacent cells.The communication module 302 receives the voltage states of the adjacentcells from the sensing module 106. The communication module 302communicates with communication modules of adjacent cell balancingmodule ICs via pins 4 and 5.

The communication module 302 transmits information relating to thevoltage states of the adjacent cells balanced by the cell balancingmodule IC 300 to the adjacent cell balancing module ICs. Additionally,the communication module 302 receives information relating to thevoltage states of cells balanced by the adjacent cell balancing moduleICs. The communication module 302 receives the information relating tothe voltage states from communication modules of the adjacent cellbalancing module ICs.

Based on the voltage states of the cells in the battery stack 102, thecontrol module 304 determines whether to activate the switching module108. Specifically, the control module 304 activates the switching module108 when the voltage difference between any two cells in the batterystack 102 is greater than a threshold.

In FIG. 4B, a cell balancing module IC 350 comprises all the componentsof the cell balancing module IC 105. Additionally, the cell balancingmodule IC 350 comprises the communication module 302 and the controlmodule 304. The communication module 302 communicates with thecommunication modules of the adjacent cell balancing module ICs via pins6 and 7. The capacitor C is located externally to the cell balancingmodule IC 350.

Referring now to FIGS. 5A and 5B, for example, systems for capacitivevoltage balancing of four cells of the battery stack 102 are shown. InFIG. 5A, a functional block diagram of a system 400-1 comprising aplurality of the cell balancing module IC 300 is shown. In FIG. 5B, afunctional block diagram of a system 200-2 comprising a plurality of thecell balancing module IC 350 is shown.

In FIG. 5A, the system 400-1 comprises first, second, and third cellbalancing module ICs 300-1, 300-2, 300-3 and Cell-1, Cell-2, Cell-3, andCell-4 of the battery stack 102. The first, second, and third cellbalancing module ICs 300-1, 300-2, 300-3 balance the output voltages ofCell-1, Cell-2, Cell-3, and Cell-4.

Specifically, the first cell balancing module IC 300-1 balances theoutput voltages of Cell-1 and Cell-2. The second cell balancing moduleIC 300-2 balances the output voltages of Cell-3 and Cell-4. The thirdcell balancing module IC 300-3 balances the output voltages of Cell-2and Cell-3.

The first cell balancing module IC 300-1 is connected across Cell-1 andCell-2 and communicates with node N1 that connects the Cell-1 and Cell-2in series. The second cell balancing module IC 300-2 is connected acrossCell-3 and Cell-4 and communicates with node N2 that connects the Cell-3and Cell-4 in series. The third cell balancing module IC 300-3 isconnected across Cell-2 and Cell-3 and communicates with node N3 thatconnects the Cell-2 and Cell-3 in series. The third cell balancingmodule IC 300-3 is connected across the nodes N1 and N2.

The first, second, and third cell balancing module ICs 300-1, 300-2,300-3 are linked via pins 4 and 5 as shown. Specifically, the first,second, and third cell balancing module ICs 300-1, 300-2, 300-3 arelinked in the form of a string or a chain as shown. The first, second,and third cell balancing module ICs 300-1, 300-2, 300-3 communicateinformation relating to the voltage states of Cell-1, Cell-2, Cell-3,and Cell-4.

For example, the first cell balancing module IC 300-1 transmits thevoltage states of Cell-1 and Cell-2 to the third cell balancing moduleIC 300-3. The second cell balancing module IC 300-2 transmits thevoltage states of Cell-3 and Cell-4 to the third cell balancing moduleIC 300-3. The third cell balancing module IC 300-3 transmits the voltagestates of Cell-2 and Cell-3 to the first and second cell balancingmodule ICs 300-1, 300-2.

Thus, each of the first, second, and third cell balancing module ICs300-1, 300-2, 300-3 receives the voltage states of Cell-1, Cell-2,Cell-3, and Cell-4. Based on the voltage states, the control modules ofthe first, second, and third cell balancing module ICs 300-1, 300-2,300-3 activate the respective switching modules. Specifically, thecontrol modules activate the switching modules when the voltagedifference between any two of Cell-1, Cell-2, Cell-3, and Cell-4 isgreater than the threshold. The threshold may be programmable.

In FIG. 5B, the system 400-2 comprises first, second, and third cellbalancing module ICs 350-1, 350-2, 350-3 and Cell-1, Cell-2, Cell-3, andCell-4 of the battery stack 102. The first, second, and third cellbalancing module ICs 350-1, 350-2, 350-3 balance the output voltages ofCell-1, Cell-2, Cell-3, and Cell-4. The capacitor C is locatedexternally to the first, second, and third cell balancing module ICs350-1, 350-2, 350-3.

Specifically, the first cell balancing module IC 350-1 balances theoutput voltages of Cell-1 and Cell-2. The second cell balancing moduleIC 350-2 balances the output voltages of Cell-3 and Cell-4. The thirdcell balancing module IC 350-3 balances the output voltages of Cell-2and Cell-3.

The first cell balancing module IC 350-1 is connected across Cell-1 andCell-2 and communicates with node N1 that connects the Cell-1 and Cell-2in series. The second cell balancing module IC 350-2 is connected acrossCell-3 and Cell-4 and communicates with node N2 that connects the Cell-3and Cell-4 in series. The third cell balancing module IC 350-3 isconnected across Cell-2 and Cell-3 and communicates with node N3 thatconnects the Cell-2 and Cell-3 in series. The third cell balancingmodule IC 350-3 is connected across the nodes N1 and N2.

The first, second, and third cell balancing module ICs 350-1, 350-2,350-3 are linked via pins 6 and 7 as shown. Specifically, the first,second, and third cell balancing module ICs 350-1, 350-2, 350-3 arelinked in the form of a string or a chain as shown. The first, second,and third cell balancing module ICs 350-1, 350-2, 350-3 communicatevoltage states of Cell-1, Cell-2, Cell-3, and Cell-4.

For example, the first cell balancing module IC 350-1 transmits thevoltage states of Cell-1 and Cell-2 to the third cell balancing moduleIC 350-3. The second cell balancing module IC 350-2 transmits thevoltage states of Cell-3 and Cell-4 to the third cell balancing moduleIC 350-3. The third cell balancing module IC 350-3 transmits the voltagestates of Cell-2 and Cell-3 to the first and second cell balancingmodule ICs 350-1, 350-2.

Thus, each of the first, second, and third cell balancing module ICs350-1, 350-2, 350-3 receives the voltage states of Cell-1, Cell-2,Cell-3, and Cell-4. Based on the voltage states, the control modules ofthe first, second, and third cell balancing module ICs 350-1, 350-2,350-3 activate the respective switching modules. Specifically, thecontrol modules activate the switching modules when the voltagedifference between any two of Cell-1, Cell-2, Cell-3, and Cell-4 isgreater than the threshold. The threshold may be programmable.

When cell balancing modules are linked as described above, the cellbalancing modules can handle a stack of cells having a voltage greaterthan a voltage that each cell balancing module can sustain. This isbecause each cell balancing module communicates with one or moreadjacent cell balancing modules. Each cell balancing module and theadjacent cell balancing modules operate at the same voltage. The voltageis the same due to overlapped cell operation in the battery stack.Further, the linked cell balancing modules operate without furtherisolation (e.g., without capacitive coupling).

Referring now to FIGS. 6A and 6B, a battery stack control module 500 maycommunicate with the cell balancing module ICs via a communication linkthat links the cell balancing module ICs. In FIG. 6A, for example, thebattery stack control module 500 communicates with pin 5 of the secondcell balancing module IC 300-2. In FIG. 6B, for example, the batterystack control module 500 communicates with pin 7 of the second cellbalancing module IC 350-2. In other words, the battery stack controlmodule 500 may be connected to one end of the communication link thatlinks the cell balancing module ICs.

In FIGS. 6A and 6B, the battery stack control module 500 controls aswitch 502. The switch 502 connects/disconnects the battery stack 102to/from a charging system and/or a load (not shown). The battery stackcontrol module 500 receives information relating to the voltage statesof the cells of the battery stack 102 from the cell balancing moduleICs. The battery stack control module 500 closes/opens the switch 502based on the voltage states to connect/disconnect the battery stack 102.

Referring now to FIG. 7, a method 600 for voltage balancing usinghysteretic capacitive charge transfer between cells is shown. The method600 may be performed during charging and/or discharging of arechargeable battery stack. Control begins at 602. At 604, controldetermines whether the voltage difference between adjacent cells isgreater than or equal to a first threshold. Control repeats 604 untilthe voltage difference between adjacent cells is greater than or equalto the first threshold.

At 606, control performs capacitive charge transfer between the adjacentcells when the voltage difference between adjacent cells is greater thanor equal to the first threshold. At 608, control determines whether thevoltage difference between adjacent cells is less than or equal to asecond threshold, where the second threshold is less than the firstthreshold. Control returns to 606 when the voltage difference betweenadjacent cells is not less than or equal to the second threshold. At610, control stops the capacitive charge transfer when the voltagedifference between adjacent cells is less than or equal to the secondthreshold. Control returns to 604.

Referring now to FIG. 8, a method 700 for capacitive charge transferwhen the number of cells is high is shown. Control begins at 702. At704, control determines whether the number N of cells in the batterystack is greater than a predetermined number. At 706, control performshysteretic capacitive charge transfer (method 600 of FIG. 7) when thenumber N of cells in the battery stack is less than the predeterminednumber.

At 708, when the number N of cells in the battery stack is greater thanor equal to the predetermined number, control receives voltage states ofthe N cells via communication links between cell balancing module ICs.At 710, control determines based on the voltage states whether thevoltage difference between any of the N cells is greater than or equalto a predetermined threshold.

Control returns to 708 when the voltage difference between any of the Ncells is less than the predetermined threshold. At 712, when the voltagedifference between any of the N cells is greater than or equal to thepredetermined threshold, control performs capacitive charge transferbetween the N cells. Control returns to 708.

FIG. 9 illustrates another system 900 for balancing output voltages ofcells in a rechargeable battery stack. The system 900 includes a cellbalancing module 904 and a rechargeable battery stack 902. The cellbalancing module 904 comprises two switches SW1 and SW2, a switchingmodule 908, a sensing module 910 and an inductive element 906.

The inductive element 906 may include an inductor or any other devicesthat can be used to effect inductance. The inductive element 906 iscoupled between a node N1 between Cell-1 902-1 and Cell-2 902-2 and anode Vx between the switches SW1 and SW2. The use of the inductiveelement 906 provides higher current handling capacity, as compared tothe use of capacitance in the system 100 described above.

The switching module 908 controls switching of the switches SW1 and SW2in response to a control signal from the sensing module 910.

The sensing module 910 senses the current across the inductive element906, or alternatively, senses the voltage at node Vx.

The system 900 performs cell balancing between Cell-1 902-1 and Cell-2902-2 as follows. In one scenario, energy is transferred from Cell-1902-1 to Cell-2 902-2. Switch SW1 is turned on and switch SW2 is turnedoff by the switching module 908. This creates a current through theinductive element 906. Then, switch SW1 is turned off and switch SW2 isturned on until the current through the inductive element 906 is greaterthan or equal to zero. Switch SW2 is then turned off once the currentthrough the inductive element 906 is greater than or equal to zero.Current through the inductive element 906 is sensed or monitored by thesensing module 910. Alternatively, a waiting period is taken untilvoltage at node Vx and voltage at node N1 are approximately equal. Then,the sensing module 910 can sense the voltage at node Vx to effect theswitching accordingly. The foregoing switching of switches SW1 and SW2can be repeated as needed.

In another scenario, energy is transferred from Cell-2 902-2 to Cell-1902-1. To achieve such energy transfer, the switching sequence asdescribed above in connection with the energy transfer from Cell-1902-1to Cell-2 902-2 is reversed.

When Cell-1 902-1 and Cell-2 902-2 are balanced, the switches SW1 andSW2 are turned off.

Based on the disclosure and teachings provided herein, it should beunderstood that the cell balancing module 904 having an inductiveelement 906 may be utilized in similar arrangements as described inconnection with the cell balancing module 104 having a capacitiveelement, e.g., FIGS. 2A, 3A, 3B, 3D, 3E, 4A, 5A and 6A.

The broad teachings of the disclosure can be implemented in a variety offorms. Therefore, while this disclosure includes particular examples,the true scope of the disclosure should not be so limited since othermodifications will become apparent upon a study of the drawings, thespecification, and the following claims.

What is claimed is:
 1. A system comprising: a rechargeable battery packincluding a plurality of cells, wherein the plurality of cells of therechargeable battery pack are connected in series; and a plurality ofcell balancing modules, wherein each cell balancing module of theplurality of cell balancing modules is configured to perform voltagebalancing of a respective pair of cells, wherein each cell balancingmodule includes a communication module configured to (i) transmit, via acommunication link, information about voltages of the respective pair ofcells to adjacent ones of the plurality of cell balancing modules, and(ii) receive, via the communication link, from adjacent ones of theplurality of cell balancing modules, information about voltages of cellscorresponding to the adjacent ones of the plurality of cell balancingmodules, and wherein in response to a voltage difference between any ofthe plurality of cells in the rechargeable battery pack being greaterthan or equal to a predetermined threshold, each cell balancing moduleis configured to perform the voltage balancing (i) based on theinformation received from the adjacent ones of the plurality of cellbalancing modules about the voltages of the cells corresponding to theadjacent ones of the plurality of cell balancing modules, instead of(ii) based on a difference between voltages of the respective pair ofcells.
 2. The system of claim 1, wherein each cell balancing modulefurther comprises: a sensing module configured to sense the voltages ofthe respective pair of cells; and a switching module configured toalternately connect a capacitance across a first cell and a second cellof the respective pair of cells at a switching frequency in response toa difference in the voltages of the first cell and the second cell beinggreater than or equal to a first threshold, and stop alternatelyconnecting the capacitance across the first cell and the second cell ofthe respective pair of cells at the switching frequency in response tothe difference in the voltages of the first cell and the second cellbeing less than or equal to a second threshold, wherein the firstthreshold is greater than the second threshold.
 3. The system of claim2, wherein each cell balancing module further comprises the capacitanceto transfer charge between the first cell and the second cell of therespective pair of cells in response to the capacitance beingalternately connected across the first cell and the second cell of therespective pair of cells at the switching frequency.
 4. The system ofclaim 3, wherein each cell balancing module is further configured toprevent overcharging and over-discharging of the first cell and thesecond cell by (i) alternately connecting the capacitance across thefirst cell and the second cell and (ii) transferring charge between thefirst cell and the second cell.
 5. The system of claim 1, furthercomprising a stack control module configured to: communicate with one ofthe plurality of cell balancing modules via the communication link,receive, from the one of the plurality of cell balancing modules,information about voltages of the plurality of cells in the rechargeablebattery pack via the communication link, and control coupling of therechargeable battery pack to a charging system based on the informationabout the voltages of the plurality of cells in the rechargeable batterypack.
 6. The system of claim 5, wherein the stack control module isconfigured to control coupling of the rechargeable battery pack to aload based on the information about the voltages of the plurality ofcells in the rechargeable battery pack.
 7. A method for balancingvoltages of a plurality of cells in a rechargeable battery pack, whereinthe plurality of cells of the rechargeable battery pack are connected inseries, and wherein the rechargeable battery pack further includes aplurality of cell balancing modules, the method comprising: each cellbalancing module of the plurality of cell balancing respectivelyperforming voltage balancing of a respective pair of cells inrechargeable battery pack; each cell balancing module receiving, via acommunication link, from adjacent ones of the plurality of cellbalancing modules, information about voltages of cells corresponding tothe adjacent ones of the plurality of cell balancing modules; and inresponse to a voltage difference between any of the plurality of cellsin the rechargeable battery pack being greater than or equal to apredetermined threshold, each cell balancing module performing thevoltage balancing based on (i) the information received from theadjacent ones of the plurality of cell balancing modules about thevoltages of the cells corresponding to the adjacent ones of theplurality of cell balancing modules, instead of (ii) based on adifference between voltages of the respective pair of cells.
 8. Themethod of claim 7, wherein performing the voltage balancing based on adifference between voltages of the respective pair of cells comprises:sensing the voltages of the respective pair of cells; alternatelyconnecting a capacitance across a first cell and a second cell of therespective pair of cells at a switching frequency in response to adifference in the voltages of the first cell and the second cell beinggreater than or equal to a first threshold; and stopping alternatelyconnecting the capacitance across the first cell and the second cell ofthe respective pair of cells at the switching frequency in response tothe difference in the voltages of the first cell and the second cellbeing less than or equal to a second threshold, wherein the firstthreshold is greater than the second threshold.
 9. The method of claim8, wherein performing the voltage balancing based on a differencebetween voltages of the respective pair of cells further comprisestransferring charge between the first cell and the second cell of therespective pair of cells in response to the capacitance beingalternately connected across the first cell and the second cell of therespective pair of cells at the switching frequency.
 10. The method ofclaim 9, further comprising preventing overcharging and over-dischargingof the first cell and the second cell by (i) alternately connecting thecapacitance across the first cell and the second cell and (ii)transferring charge between the first cell and the second cell.
 11. Themethod of claim 7, further comprising: communicating with one of thecell balancing modules via the communication link; receiving, from theone of the plurality of cell balancing modules, information aboutvoltages of the plurality of cells in the rechargeable battery pack viathe communication link; and controlling coupling of the rechargeablebattery pack to a charging system based on the information about thevoltages of the plurality of cells in the rechargeable battery pack. 12.The method of claim 11, further comprising controlling coupling of therechargeable battery pack to a load based on the information about thevoltages of the plurality of cells in the rechargeable battery pack.